This invention relates to control of the excess air factor of an internal combustion engine.
Tokkai Hei 11-294145 published by the Japanese Patent Office in 1999 discloses a catalyst which accumulates nitrogen oxides (NOx) generated by a diesel engine of a vehicle when it is operated at a lean air/fuel ratio, in other words at a large access air factor. The catalyst is provided in the exhaust passage of the engine, and the NOx which is accumulated by the catalyst is eliminated from time ti time by operating the engine at a rich air/fuel ratio.
When the engine is operated at a rich air/fuel ratio, hydrocarbons (HC) are generated. The NOx accumulated in the catalyst is reduced by these hydrocarbons, and is released into the atmosphere as innocuous nitrogen (N2). By this elimination of NOx, the catalyst is regenerated back into its state in which it fresh air intake flow amount Qas0 can resume accumulation of NOx. Temporarily enriching the air/fuel ratio in this manner, in other words temporarily reducing the access air factor below unity, is termed a xe2x80x9crich spikexe2x80x9d. Such a rich spike is generated by reducing the intake air amount of the engine while increasing its fuel injection amount.
When a rich spike is generated, because of a reaction lag which depends upon the volume of the air intake system, although the fuel injection amount increases instantaneously, the intake air amount does not diminish immediately. As a result, a temporary sudden increase of the output torque of the engine may occur during a rich spike. Furthermore, for an engine in which the excess air factor is controlled based upon a target excess air factor, variation of the output torque of the engine can easily occur when altering the target excess air factor, due to a lag between the control response of the fuel injection amount and that of the intake air amount, not only in the case of a rich spike which is generated for regenerating the NOx catalyst, but in other cases as well. However, it is not desirable for the output torque of the engine to vary independently of the operation by the driver of the vehicle.
It is therefore an object of this invention to suppress variation of torque caused by alteration of the target excess air factor.
It is a further object of this invention to converge the excess air factor to the target excess air factor in a short time while suppressing the engine output torque when the target excess air factor has changed.
In order to achieve the above objects, this invention provides a device for controlling an excess air factor of an air-fuel mixture which is supplied to an engine. The device comprises a sensor which detects an operation condition of the engine, a fuel supply mechanism which supplies fuel to the engine, a sensor which detects a real fresh air intake amount of the engine; and a programmable controller. The programmable controller is programmed to set a basic target excess air factor based upon the operation condition, set a target fresh air intake amount based upon the operation condition, calculate a target excess air factor by correcting the basic target excess air factor based upon the target fresh air intake amount and the real fresh air intake amount, calculate a target fuel supply amount based upon the target excess air factor, and control a fuel supply amount of the fuel supply mechanism to be equal to the target fuel supply amount.
This invention also provides a method for controlling an excess air factor of an air-fuel mixture which is supplied to an engine by using a fuel supply mechanism which supplies fuel to the engine. The method comprising detecting an operation condition of the engine, detecting a real fresh air intake amount of the engine, setting a basic target excess air factor based upon the operation condition, setting a target fresh air intake amount based upon the operation condition, calculating a target excess air factor by correcting the basic target excess air factor based upon the target fresh air intake amount and the real fresh air intake amount, calculating a target fuel supply amount based upon the target excess air factor, and controlling a fuel supply amount of the fuel supply mechanism to be equal to the target fuel supply amount.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.